Mitochondrial complex I (CI) activity appears to play a key role in the pathogenesis of Parkinson?s disease (PD). Cl activity is impaired in the substantia nigra (SN) in PD, and Cl inhibitors induce parkinsonism when systemically administered in animals. Indirect evidence suggests a role for mitochondrial DNA (mtDNA) mutations. However, detection of these mutations may not be possible by standard screening techniques, particularly if they are present at low mutational burdens. We hypothesize that numerous acquired mutations, each individually present at a low mutational burden, could reach a sufficient aggregate burden to cause mitochondrial dysfunction. Such mutations are hypothesized to result from oxidative damage to mtDNA. Brain levels of 8-hydroxy-2?deoxyguanosine (OH8dG), a marker of oxidative DNA damage associated with point mutations, are 16-fold higher in mtDNA than in nuclear DNA, increase with aging, and increase further in PD. We have developed a protocol for detecting mtDNA mutations present at extremely low mutational burdens. We propose to determine the frequency of oxidative stress-induced mtDNA mutations in frontal cortex and substantia nigra in controls and in PD. Using Laser Capture Microdissection (Arcturus), we will examine the specific subpopulation of neurons susceptible in PD. Single-cell PCR will allow us to address the question of accumulation of individual acquired mutations within single neurons. We also propose to establish an in vitro system for analyzing the induction of oxidative stress-induced mutations in dividing and in post-mitotic cells by exposure to hydrogen peroxide or to agents that inhibit mtDNA repair. Collaborations have been established which will make the proposed studies possible. These studies will lay the foundation for future studies addressing the role of acquired mtDNA mutations in aging and in neurodegenerative diseases.